1. Technical Field
This disclosure relates to integrated circuits, and more particularly to integrated circuit security.
2. Description of the Related Art
Integrated circuits (ICs) are commonly involved in various secure transactions or computations. For example, encryption is used to keep secret data or messages that are used in financial transactions, personal communications, and communications relating to national security. Great care is taken to protect this information, and conversely, malicious persons or organizations expend great effort to uncover these secrets.
ICs may provide security in part because of their very small size and the difficulty of directly accessing electrical signals inside the IC. However, attackers have developed and continue to develop a variety of methods to infer the data contained within or the computations being performed inside ICs. These methods include analyzing power consumption variation, monitoring electromagnetic emissions, partial deprocessing of the IC, mechanical probing, and many others.
A typical modern IC may have numerous layers of wiring overlaying the transistors. Access to any particular wire is usually blocked by overlaying wires. This is a problem not only for malicious attackers, but for chip debug and failure analysis. Consequently, analysis techniques have been developed that permit access to transistors and nodes from the backside of the IC. One such analysis technique is known as back side infrared (IR) probing. Since silicon is transparent to IR light, IR lasers may be used to penetrate the back side of the IC and to probe the transistor structures within the silicon. As shown in FIG. 1, a prior art IC 20 in a package 10 is being electrically probed on the back side 25 by an infrared laser probe 30. The IC is mounted in the package in a flip-chip configuration such that the top layer metal is on the bottom as drawn and coupled to the package using solder balls 12. The IR laser probe 30 produces an IR beam 35 which penetrates through the back side silicon 25 to nodes formed on the front side 28 of the silicon.
A number of back side probing techniques have been developed. Some allow observation of voltages or voltage waveforms, while other techniques allow the function of transistors to be modified via IR light. Attackers may employ these techniques to determine the nature of the chip operation and to discover the data present in the IC at any particular time.
The backside probing method that provides the greatest amount of detail in terms of time-varying signal information is the laser voltage probe. This method generally requires that the IC operation is repeated many times, and the signal waveform is acquired by repeatedly sampling one or more electrical nodes (usually a transistor source or drain). Repeated probing is required both to increase the signal-to-noise ratio of the acquired signal, and to sample the signal at multiple points in time relative to the sequence of events in the IC. An externally accessible timing event such as reset for example, may be used as a reference point in determining exactly at what time to take the next optical signal sample. Because each sample may have a very short duration and may have a low signal-to-noise ratio, many samples may be needed to build a waveform with an interesting length of time.